Ultra high frequency converter circuit



March 6, 1945. l. E. THOMPSON 4 2,370,758

ULTRA-HIGH FREQUENCY CONVERTER CIRCUIT 7 Filed April 3, 1943 (2W ZF=304/6:

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pom 7 0F I l I INVENTOR laA/vo E ZZ/OMPJON ATTORN EY Patented Mar. 6,1945 ULTRA men FlCE I Leland E. Thompson, Merchantville, N. J., assignorto Radio Corporation of America, a corporation of Delaware ApplicationApril 3, 1943, Serial No. 481,687

20mins.

' My present invention relates to ultra-high frequency convertercircuits, and more particularly to an ultra-high frequency triodeconverter circult. Y

' As is well known, there are special problems involved in the receptionof radio frequency signals in the ultra-high frequency band. Oneof themost important problems which confronts the designer of an ultra-highfrequency receiver is to providea high sensitivity at the convertercircuit itself. In a receiver utilizing no radio frequency amplificationprior to the input circuit of the converter, and covering, for example,a frequency range of 225 to 400 megacycles (me), it

is also especially important to provide a high signal-to-noise ratio atthe converterinput circuit.

I have found in an ultra-highfrequency converter. circuit utilizing atriode tube that increasing the magnitude of the inductance in theoathode lead, will materially increase the sensitivity and thesignal-to-noise ratio of the converter up to a certain inductance value."Ihave found that beyond this value the sensitivity drops very rapidly.It is one of the main objects of my present invention to provide anultra-high frequency converter with an increase in sensitivity andsignalto-noise ratio of at least two times. This has been obtained in anactual receiver by maintaining the converter cathode circuit at apredetermined inductive value.

- It is another important object of this invention to provide anultra-high frequency receiver adapted to operate to receive signalswhose freq y is as high as mo. (megacycles), the receiver having itssignal collecting device coupled directly to the tunableinput circuit ofthe converter, and the local oscillator being coupled to an inductanceof predetermined magnitude in the cathode lead of the converter tube.

Still other objects of my invention are to improve generally thesensitivity and efficiency of ultra-high frequency converter circuits ofthe triode type, and more particularly to provide a simple andefiectivemethod of securingan appreciable increase in signal-to-noise ratio insuch a converter circuit.

The novel features which I believe to be characteristic of myinventionare set forth in particularity in the appended claims;v theinvention,

Fig. 2 is a purely qualitative curve to illustrate the advantage of theinvention.

In Fig. l'there is shown a triode I which is used as the converter tubeof the ultra-high frequency receiver to which this invention isdirected. The tube l is schematically represented,-but it will beclearly understood that it will be of a type suitable for functioning asaconverterin the ultrahigh frequency band of operation of this receiver.For example, a type 955 triode could be used. Let

-' it be assumed that the'desired frequency band is of the order of 225to 400 inc., and that the receiver is of the superheterodyne type. The,Dreliminary resonant selector circuit consists of a resonant linecircuit employing capacitytuning. The numeral 2 denotes a one-quarterwave length concentric line of any well known form. The numeral 3designates'the capacity tuning means. It will be understood that anactual physical condenser need not beemployed, since any well knowndevice can be used to vary the capacity of the resonant line 2. Thenetwork 2-3, then, pro-; vides a tunable selector circuit adapted to beadjusted over the given frequency range of 225 to 400 me. The cathode 4of tube l is connected toground'through an inductance coil L. The control grid 5 is connected to the high potential side of'circ'uit 2-3through a grid coupling condenser 6. The numeral 1 designates the usualgrid return resistor connected from grid 5 to ground. The plate 8 isconnected to the +3 terminal of the direct current energizing sourcethrough a path comprising the primary winding 9 of intermediatefrequency output transformer 10 and the resistor II. I

The resistor H is bypassed to ground for-the ultra-high frequencycurrents by condenser l2.

The. condenser i3 is connected to ground from the plate end of coil 9.The function of condenser 13.

is to bypass ultra-high frequency currents, and

. also to act as a tuning capacitor for the primary circuit 13-9 will betuned to the operating 1. F.

value, and the latter will'have any desired value.

e For example, an I. F. of 3B mc., could be used.

The numeral I4 designates the secondary circuit of the I. F. transformerl0, and the latter will also be tuned to the operating 1. F. value. Thesubsequent networks are not shown, because they are not a part of thisinvention. It is sufficient to point out that the usual networks of asuperheterodyne receiver may be employed subsequent put coil I! of thelocal oscillator is shown magnetically coupled to the cathode coil L..It will,

therefore,-be seen that the local oscillations areinjected into thecathode circuit of the converter ous frequency ranges are purelyillustrative, and

outthat some benefit is obtained by the adding of inductance in thecathode circuit long before the oint of optimum operation is reached.For example, at 400 megacycles the inductance L acare in no senserestrictive as to the scope of my invention.

The specific antenna, or signal collecting device, is not shown, but maybe a dipole'which is magnetically coupled to the tunable input circuit2-3 as schematically shown. If desired, there 'includethe cathode leadinside of tube 1. '10

tually consisted of a wire between 2 and 3 inches long in the form of aloop. This same inductance gave some benefit at a frequency of 200megacycles. The total inductive magnitude of L would The type of tubeused would then have some effect on how much external inductance shouldbe added.

- The inductance L would probably be of the order can be employed aco-axial cable which is adapted to feed the ultra-high frequency signalenergyto the converter input circuit. The coil L may be made to have anadjustable value, if desired. It can be a fixed inductance of the propervalue to obtain the desired sensitivity. The converter tunedinputcircuit, which is of the resonant line type, is a high Q circuit with a.consequent high resonant impedance. The coil L may be composed of twoseparate inductances in series, if de-- sired; one part is used tocouple to the local oscillator, and the other part is predetermined .invalue so that the total cathode inductance will provide the beneficialresults desired. It should be understood that while the cathode circuitiniection of the local oscillator is convenient and effective, the localoscillator, if desired, may be coupled to the grid circuit.

I have found in actual experience with the circuit of Fig. 1 that thereis .a point of optimum operation for the converter circuit. That is tosay, with a predetermined value of inductance L inserted in the cathodecircuit, the signal-to-noise ratio will be a maximum value. Forinductance values less than the critical value which has beenpredeterminedthe sensitivity of the converter decreases. Furthermore,for. inductance values beyond the critical, value the sensitivity dropsvery rapidly. In Fig. 2 there is depicted, in a purely qualitativemanner, that by-assigning the critical value of the inductance L theremay be secured a greatly increased signal-to-noise ratio. Indeed, forthe range :of 225 to 400 megacycles an increase, in sensitivity andsignal-to-noise ratio of; two times has been obtained by adding. theproper amount ofinductance Lin the cathode lead. It has been found byactual experiment that the results obtained are not due to an. increaseor-decrease in oscillatorexcitation voltage impressed. on the converter.I do not wish to limit myself to any particular theoretical explanationwhich would explain the reason for this rise. in sensitivity, sinceactual experience with; the circuit has demonstrated that such a sharprise in sensitivity issecured. What ismost im'-- portant-in the presentcircuit is that the-increase in gain at the converter circuit. givesanincrease in signal-to-noise ratio of an. appreciable amount (about 6decibels) which makes itparticularly valuable inan ultra-high frequencyreceiver;

1 may point out that I have found it necessary that the cathode circuitof the convertenbe in:

ductive at the selected signal: frequency'to-obtain the beneficialeiiectswhich I. have observed. The? selectivity of the receiver isdependent entirelyon'.

the I. F. band width, and not on the input-circuit:

of the converter stage. Therefore, theefiects described herein'cannotbe' said to be due'to'a reduction in band width at the converter inputcir cult. I do not wish it to. be-understcodith'at. the

adjustment of the. cathode? circuit-inductance. L. issharply critical,In; thatregard iris-1: pointed of 0.1 microhenry.

It may well be that the results obtained with this converter circuit maybe due to the inductance in the cathode lead compensating, or in someway rendering less eifective, the electron transit time of the convertertube. The input conductance formula for avacuum tube is usually givenfor the case where the inductance of the cathode lead is purposely keptsmall, This formula shows that theinput resistance is higher whenL islow. Considering the voltages appear-- ing fromgrid 5 to ground of thetube I, due to signal voltage input the vector relations of thesevoltages show that it is possible to adjust hot the cathcdecircuit sothat it is possible to make the maximum charge appear on the grid 5 atthe time of maximum signal voltage. That is, the

alternations of the charge on the grid 5 may be made to be in phase withthe signal alternating current. This condition appears to eliminate theeffect of electron transit time. Indeed, this effeet becomes a factor ofincreasing importance tion is by no means limited. to the particularorganization shown and described, but that-many modifications may bemade without departing from the scope of my invention, as set forthinthe appended claims.

What I claim is:

1. In an ultra-high frequency converter stage of the type comprising aconverter tube of the triode type provided with atunable input circuitadjustable over a range of 200 to 400 megacycles,

- andhaving a local oscillator constructed and arranged to inject-itsoscillationsinto'the converter tube cathode circuit; the improvementwhich includes the direct connection of an inductancebetween theconverter'tube cathode and grounclfor rendering the: cathode circuitessentially inductive, and. said inductance having a predetermined valuet greatly increase the sensitivity of the converter stage.

2. A method of compensating.for electron transittime'ofia.c'o'nver-ter'tube operating: in the ultra-- high frequency'rang'eandhaving its signal inputcircuit directly coupled to-asignal. collectorde==v vice;: thei' method comprising 'the-stepsof inject-'- ing; into:the converter cathode circuit locallyproduced: oscillations, and:maintaining the-cath ode: circuit-essentially inductive and of a prede=-termined magnitude such as to provide the said electron transit timecompensation whereby the sensitivity of the converter greatlyincreased.- Eb THGM-PSONr-

